Heating systems and heater units therefore

ABSTRACT

An oil heater unit which comprises a chamber through which oil is intended to flow; a closed vessel mounted in a wall of the chamber, the vessel having a first portion situated inside the chamber and a second portion situated outside the chamber and being so arranged that material contained in the vessel can flow from the first portion to the second portion; a heater device for heating the second portion of the vessel; and wherein the vessel contains a heat transfer material which is adapted to wholly or partially vaporize in the closed vessel when the second portion thereof is heated whereby the vapor transfers heat to the first portion of the vessel by which the chamber and oil flowing therethrough is adapted to be heated at least to a temperature substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determined within the closed vessel.

United States Patent [I91 Asdell et a]. I

[ Apr. 16, 1974 1 HEATING SYSTEMS AND HEATER UNITS THEREFORE [73] Assignee: Churchill Instrument Company Limited, Perivale, Greenford, Middlesex, England [22] Filed: Dec. 20, 1971 [21] Appl. No.: 209,895

[30] Foreign Application Priority Data .Ian. 1, 1971 Great Britain 109/71 [52] US. Cl 165/1, 122/33, 126/3435, 7 165/105, 165/147 [51] Int. Cl. F28d 15/00 [58] Field of Search 126/3435 A, 351; 122/33; 165/105, 147, l

[56] References Cited UNITED STATES PATENTS 529,304 11/1894 Perkins 122/33 2,495,924 1/1950 Foley 126/351 X Primary Examiner-Carroll B. Dority, Jr.

Assistant Examiner-Peter D. Ferguson Attorney, Agent, or Firm-Stevens, Davis, Miller & Mosher [5 7] ABSTRACT An oil heater unit which comprises a chamber through which oil is intended to flow; a closed vessel mounted in a wall of the chamber, the vessel having a first portion situated inside the chamber and a second portion situated outside the chamber and being so arranged that material contained in the vessel can flow from the first portion to the second portion; a heater device for heating the second portion of the vessel; and wherein the vessel contains a heat transfer material which is adapted to wholly or partially vaporize in the closed vessel when the second portion thereof is heated whereby the vapor transfers heat to the first portion of the vessel by which the chamber and oil flowing therethrough is adapted to be heated at least to a temperature substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determined within the closed vessel.

3,321,002 5/1967 Winkler 2,153,942 4/1939 Spalding, Jr. 165/105 FOREIGN PATENTS OR APPLICATIONS Chm, 7 Draw";

407,049 3/1934 Great Britain 126/3435 I 5 l0 1 9 l.i j l,

ATENTED APR 16 I974 SHEET 3 0F 5 PATENTEHAPH 16 I971? 3894154 sum 5 OF 5 FIG.7

HEATING SYSTEMS AND HEATER UNITS THEREFORE This invention relates to an oil heating system and an oil heater unit for natural, synthetic, vegetable or mineral oils and is particularly concerned with an oil heating system and an oil heater unit by which heat may be transferred to oil flowing through a closed heating cir cuit.

lt is well known in industry that a considerable number of oils although providing an efficient heat transfer medium, have a maximum temperature to which they can be subjected without undergoing degradation. This is especially true, of petroleum oils and cooking oils where it is found that, provided heat transfer to the oil is maintained below a predetermined maximum, many oils provide an efficient heat transfer medium over a wide temperature range. However-if the heat transfer is such as to elevate the temperature of all or any part of the oil above a predetermined maximum temperature cracking is experienced and the oil breaks down.

For reasons of economy, it is sometimes recommended that heat transfer oil in a heating circuit is.

heated by gas or oil firing and although electrical heating has been proposed the running costs are generally high. With oil or gas firing, direct flame or high temperature gasimpingement on the heating surfaces with which the oil is in contact is difficult to avoid and various systems have been devised in an attempt to ensure that the temperature of the heat transfer surface in contact with the oil is not excessive. For example, thermostatically controlledcircuits have been incorporated in the system to ensure that if the temperature of the oil becomes too high, the temperature of the heat sources is reduced accordingly, and high speed pumps, agitators and baffles may be provided in the circuit of the system to ensure that the flow speed of the oil is maintained at a high rate and under turbulent conditions so that there is little or no opportunity for the oil to dwell on a possible hot-spot in the system. Even with these precautions however, it is possible and in practice does happen, that undesirable localized heating of the oil can develop, for example at a hot-spot in the circuit if the flow rate of the oil is reduced without permitting a corresponding reduction in the temperature of the heat source. The development of .such a hot-spot can lead to degradation of the oil as aforementioned and in an attempt to overcome this problem safeguards have to be introduced into the heating system to ensure that the temperature of the heat transfer surface in contact with the oil is not excessive. The provision of such safeguards requires the use of sophisticated control apparatus and circuits, large pumping capacities at high flow rates to produce turbulence and complex fluid flow control means andngenerally results in an expensive heater system and components.

:It is an object of the present invention to provide an oil heater unit and oil'heat'ing method and system by which the aforementioned disadvantages associated with hitherto proposed systems will substantially be avoided and which oil heating system and :oil heater unit may be of relatively simple construction and susceptible to manufacture :at relatively .low cost.

According to the present invention there :is .provided an oil heater unit which comprises a chamber through whichoil is intended to flow; a closed vessel mounted in a wall of the chamber, said vessel having a first porsituated outside the chamber and being so arranged that material contained in the vessel can flow from the first portion to the second portion; heater means for heating the second portion of the vessel; and wherein the vessel contains a heat transfer material which is adapted to wholly or partially vaporize in the closed vessel when the second portion thereof is heated whereby the vapor transfers heat to the first portion of the vessel by which the chamber and oil flowing therethrough is adapted to be heated at least to a temperature substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determined within the closed vessel.

The invention further provides an oil heating system which includes the oil heater unit constructed in accordance with the immediately preceding paragraph and in which the chamber thereof forms part of a closed circuit containing the oil to be heated. A circulating pump is provided in the circuit.

Still further according to the present invention there is provided a method of heating oil which flows through a chamber and which includes positioning in the chamber a closed vessel containing a heat transfer material and locating such vessel so that a first portion thereof is positioned within the oil flow and a second portion thereof is positioned remotely from the oil flow said portions being arranged so that the heat transfer mate rial in the vessel can flow from the first portion to the second portion thereof; and heating the second portion of the vessel to wholly or partially vaporize the heat transfer material therein whereby the vapor transfers heatto the first portion of the vessel and thereby to the oil at a temperature at least substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determinedwithin the closed vessel.

The heat transfer material as used in the closed vessel may be selected from many substances, however, due to physical limitations, it is likely that only certain materials are of immediate facility. Generally the heat transfer material will be liquid although a normally solid material may be used which, on heating, passes through a liquid phase to the vapor phase (or even sublimates) provided it is ensured that cyclic heating and giving up of latent heat, condensation and/or solidifi-- cation and reheating can take place. A convenient heat transfer material is water and, for convenience of description, it will hereinafter be referred to as such. It is to be realized however that the present invention is in no way restricted to the use of water as the heat transfer material and that other suitable materials may be used as will readily be apparent to persons skilled in the art and having an understanding of the presentinvention.

Oil flow through the chamber is preferably arranged to contact directly the outer surface of the first portion of the vessel and .to provide efficient heat transfer be tween the oil and the first portion of the vessel. The latter may have external ribs or fins to increase the contact area with, or the turbulence of, the oil. As the second .portion of the vessel (which second portion mayalso be ribbed or finned .to increase heat'transfer) :-is:heate.d, the water therein 'vaporizes and-transfers heat to the first portion of the vessel and thereby to the oil.

By the oil taking up heat from the first portion of the vessel, the vapor therein is caused to condense and return to the second portion of the vessel where it is again heated. When the temperature of the second portion of the vessel is raised sufficiently by continued heating, all the liquid water therein vaporizes at a temperature consistent with the vaporization of the water under the pressure conditions determined by the volume enclosed by the vessel and volume of water initially deposited therein. Depending upon the volume enclosed by the closed vessel and the water initially deposited therein, the maximum temperature attainable by the first portion of the vessel is (with an exception later described) substantially the temperature at which all the water in the vessel has just vaporized. As the latent heat of vaporization is drawn off from the first portion of the vessel, the vapor condenses and the water flows to return to the second portion of the vessel wherein it is again heated. In this way, and provided that the heater means continuously applies heat to the second portion of the vessel, the water/stream phases in the vessel cause it to act substantially as a thermostat to ensure that the temperature applied to the oil in the chamber cannot (for practical purposes) increase beyond a predetermined maximum. As aforementioned, this predetermined maximum temperature will depend upon the volume of the vessel and the mass of water initially deposited therein and these proportions may be varied as required to change the maximum temperature available at the first portion of the vessel. The critical temperaturefor water is approximately 371 C and this is the 'upper limit to a range of predetermined maximum temperatures which can be attained for the first portion of the vessel by use of different proportions for the volume of water and the volume enclosed by the vessel. Naturally if heat transfer materials other than water are utilized in the vessel then the upper limit to the range of predetermined maximum temperatures which can be attained for the first portion of the vessel can be changed in accordance with the critical temperature of such other heat transfer materials. For example, the maximum temperature to which a common type of heat transfer oil maybe subjected without causing serious degradation is in the order of 350C. Then, in such a case the mass of water in the vessel and volume enclosed by the vessel may be determined so that all the water just vaporizes when the pressure in the vessel is consistent with the temperature of the vapor attaining, say, 330C.

It is realized that heat will be applied to the chamber and oil therein directly through the material of the vessel as a result of conduction from the second portion to the first portion and, when the water has wholly vaporized in the vessel, as a result of convection currents in the vapor. However, such heat transfer by conduction and convection may be considered negligible (for practical purposes) in comparison with the latent heat evolved as the vapor condenses to water.

Although the vessel and water therein act as a thermostat in the oil heating system when the water has wholly vaporized, auxiliary thermostatically controlled means may be arranged in the unit or system so that when the oil or first portion of the vessel attains a predetermined temperature below the maximum temperature attainable when the water has wholly vaporized (that is when the water is only partially vaporized), the heat input from the heater means to the second portion of the vessel is automatically controlled to maintain the system in equilibrium whereby water vapor is substantially continuously being evolved and condensed from a mass of water in the vessel while the second portion thereof is maintained substantially at a constant temperature.

Preferably the closed vessel is in the form of a cylindrical tube having its ends sealed. In such form the closed vessel may be likened to a Perkins steam tube which has hitherto been proposed for use in the heating of bakers ovens to avoid exhaust flue gases resulting from combustion of solid fuel from coming into contact with the produce being baked. Conveniently, when the closed vessel is a cylindrical tube it is mounted with its axis substantially vertical so that the upper portion of its length extends into the oil flowchamber through a wall thereof and the lower portion of its length may extend into a heating chamber. Such heating chamber can be positioned adjacent to, or remote from, the oil flow chamber and is-provided with a convenient form of heater means. Alternatively, the tube may be inclined relative to the vertical or may even be horizontal provided however that the vapor, after condensing in the first portion of the tube can flow to return to the second portion of the tube.

The heater means may provide direct or indirect heating for the second portion of the vessel. For example, a form of direct heating may comprise an electrical coil, a gas, or an oil fired heater located adjacent to the second portion of the vessel. Alternatively, a preferred form of indirect heating for the second portion of the vessel comprises a stream of hot gas which flows over said second portion as a result of combustion in a suitable burner (for example from a propane/air burner constructed in accordance with the disclosure in British Patent Application No. 46523/67). Usually an array of several closed vessels will be provided in the oil flow chamber and the aforementioned stream of hot gas flowing through a heating chamber of the unit provides a convenient means of simultaneously heating all the second portions of the'array of vessels.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings in which:

FIG. 1 is a side elevation of an oil heater unit constructed in accordance with the invention;

FIG. 2 is a plan view of the unit shown in FIG. 1;

FIG. 3 is a side elevation, in greater detail, of the oil flow chamber and closed vessels associated therewith for the unit'shown in FIG. 1;

FIG. 4 is a plan view of the oil flow chamber shown in FIG. 3 and illustrates the disposition of the closed vessels;

FIG. 5 is an end elevation of the oil flow chamber shown in FIG. 3;

FIG. 6 is a section of the oil flow chamber and closed vessels taken on the line AA of FIG. 3, and

FIG. 7 diagrammatically illustrates a simple form of oil heating system constructed in accordance with the present invention, said system including the oil heater unit shown in FIG. 1.

The oil heater unit shown generally in FIGS. 1 and 2 comprises an oil flow chamber 1 having a base plate 2. The base plate 2 partitions the oil flow chamber from a heating chamber 3 which underlies the oil flow chamber. The underside of the base plate 2 in the chamber 3 is lagged (as are the other internal walls of the heating chamber 3) with a heat insulating material (2A). The base plate 2 is horizontally disposed and the chambers 1 and 3 are conveniently made in mild steel. Located in an end wall 4 of the heating chamber 3 is a gas fired burner 5 and the heating chamber 3 is tapered so that the volume enclosed thereby reduces in cross-sectional area from the end wall 4 towards an end wall 6 which is oppositely located to the end wall 4. The heating chamber 3 communicates at a position adjacent the end wall 6 with an exhaust passage 7 by which hot exhaust gases from the burner 5 are passed over the oil flow chamber 1 to a flue 8. The oil flow chamber 1 has an oil inlet 9 and an oil outlet 10 by which the heater unit is intended to be coupled in an oil heating system for example, as shown in FIG. 7. The oil heater unit is enclosed within a jacket (not shown) of heat insulating material.

The burner 5 operates a mixture of propane gas and air which is fired at a relatively large area outlet from the burner located in the heating chamber 3. The burner comprises a matrix of fibrous ceramic material through which the air and propane gas are passed to provide an intimate-mixture which is fired at its outlet. The relatively large outlet of the burner ensures that although the pressure of the gas mixture at the outlet may be low and a small flame is produced at the outlet, the heat output from the burner may be considerable. Although propane gas is preferred for use with the burner since it provides a relatively clean flame, other combustible gases may be used. I

The base plate 2 carries an array of closed sealed vessels which extend therethrough to project partly into the flow chamber 1 and partly into the heating chamber 3 and such sealed vessels will now be described, more particularly, with reference to FIGS. 3 to 6. Each closed vessel comprises a cylindrical low-carbon steel tube 11 which is sealed at its ends by low carbon steel vided, however for convenience of illustration all the sealed tubes 11 have not been shown in the drawings (although their location is indicated by the crosses in FIG. 4 and the array of vertical center lines disposed between the two tubes illustrated in FIG. 3).

Prior to being sealed, each of the tubes 11 is charged with a predetermined volume of water 13. In the present embodiment each of the sealed tubes 11 encloses a similar volume and each tube is charged with a similar volume of water. Although it may not be necessary when using water as the heat transfer material, for some other heat transfer materials it may be desirable to charge and seal the tubes 11 under vacuum.

As above mentioned, .the burner 5 provides a small flame and this is located so that there is no impingement of the flame on the tubes 11. The lower portions of the sealed tubes are heated by exhaust gases which flow from the burner 5.

The oil flow chamber 1 has its inlet 9 and outlet 10 coupled to a fluid heating system which in the embodiment shown in FIG. 7 comprises a simple oil filled closed circuit system having a circulating pump by Y which the oil is passed through the flow chamber 1 and thereafter through a heat transfer chamber 101 prior to returning to the flow chamber 1. The heat transfer chamber 101 will comprise a desired apparatus or member to be heated, as, for example, a press, oven, reactor, or bath. The system is provided with an oil reservoir 102 from which it is filled (and any losses due to leakages are replenished) by way of a branch conduit 103 from the closed circuit. The oil is heated in passing through the chamber 1 and over the upper portions of the sealed tubes 11 (as will be described below) and, to promote cross flow of the oil in the flow chamber 1, baffle plates 14 are provided in the chamber (see FIG. 4).

We will now consider operation of the heater unit with oil flowing through the flow chamber 1. As the lower portions of the sealed tubes 11 are heated by exhaust gases from the burner 5, the water 13 vaporizes. The water vapor enters the upper portion of thesealed tubes and its latent heat of vaporization is given up to the colder oil in the flow chamber 1, thereby causing the water vapor to condense and flow back into the lower portions of the sealed tubes. As the temperature of the lower portions of the sealed tubes 11 increases and the water vaporizes, the pressure in the sealed tubes 11 increases; consequently the temperature at which the water vaporizes increases accordingly. However, while water vapor is evolving at ever increasing temperatures from the water 13, the latent heat of vaporization is being transferred to the oil and this process continues so long as there is water present in the sealed tubes. When all the water in the sealed tubes has vaporized, it will be apparent-that if the temperature of the oil has not reached the temperature at which the water vaporized (under the pressure conditions determined by the volume enclosed in each sealed tube 11 and the mass or volume of water therein) the water vapor will be condensed and re-vaporized continuously to give up its latent heat of vaporization until the temperature of the oil attains that of the water vapor and a condition of equilibrium is obtained. Under such a condition of equilibrium each sealed tube and water enclosed thereby effectively constitutes a thermostat,

which acts to maintain the oil at a temperature at which all the water in the sealed tube just vaporizes. Consequently, by charging each of the sealed tubes with a predetermined volume of water for a given volume enclosed by the sealed tube, the maximum temperature to which the oil may be heated can be determined. The type of oil used in conventional heating systems may undergo degradation and crack at temperatures in the order of 350C so the volume enclosed by the sealed tubes and the volume of water therein may be arranged so that all the water vaporizes at pressure conditions consistent with a temperature of say, 330C, thereby providing a margin of safety for the oil heating. In practice it has been found that the heat transferred to the oil as a result of conduction through the material of the tube from the heating chamber 3 to the flow chamber 1, and as a result of convection through'the water vapor is negligible in comparison with the heat transferred to the oil by latent heat evolved during condensation of the water vapor. By charging the sealed tubes with different volumes of water it will readily be appreciated that the maximum temperature to which the oil may be subjected can be predetermined in a range which extends from approximately 100C (or below if the tubes are charged under vacuum) to a temperature slightly below the critical temperature for water (which is approximately 37lC). Naturally, this range may be varied by selecting a heat transfer material other than water.

The most advantageous feature of the oil heater unit above described and illustrated is that the sealed tubes 11 and water therein effectively provide a thermostat which ensures that oil in the flow chamber 1 can be heated to, and maintained at, a temperature below its degradation temperature. By this arrangement even if oil flow through the circuit ceases, static oil which is in the flow chamber 1 in direct contact with the upper portions of the sealed tubes is not raised to a temperature at which it cracks.

For an oil heating system of the type above described with reference to FIG. 7, it may not always be required for the temperature of the oil to be raised-to the maximum temperature of the system (which is predeter-' mined by the temperature at which all the water in the sealed tubes has vaporized). With this in mind a thermostatic device 105 may be incorporated in the oil flow circuit adjacent to the outlet 10. The device 105 forms part of a heater control system 106 which also includes a valve device 107. The valve device 107 controls the gas flow to the burner 5. The heater control system 106 operates so that when the oil attains a predetermined temperature (below its maximum) as detected by the thermostatic device 105, the valve device is caused partly to close and reduce the gas flow to burner 5. The heat input from the burner to the heating chamber is thereby automatically reduced to maintain a state of equilibrium at which the latent heat given up by the water vapor in condensing is just sufficient to keep the oil at its predetermined temperature. In this way water may always be present in the sealed tubes 11 although part of the volume thereof is continuously being vaporized and condensed.

Specific examples of heat transfer materials other than water which are believed to be suitable for use in the sealed tubes 11 are: an alloy of Sodium and Potassium, Sodium, Sulphur, Mercury and Fluorocarbons such as: .Dichlorodifluoromethane, Trichloromonofluoromethane, Trichlorotrifluoroethane.

In a particular example of the above described and illustrated embodiment, each closed vessel comprising a tube 11 has an external diameter of approximately 1.35 inches, an internal diameter of approximately 0.85 inches and an internal axial length of approximately 10.25 inches. Each such vessel contained approximately mls. of water and was mounted in the base plate 2 so that it extended approximately 6.5 inches into the oil flow chamber 1 and approximately 3.75 inches into the heating chamber 3. These vessels were found suitable for heating oil up to 300C although all the water will not vaporize until a temperature of approximately 310C is reached.

We claim:

1. A method of heating oil which flows through a chamber, said oil commencing degradation at a predetermined temperature and which method includes the steps of positioning in the chamber a closed vessel containing a heat transfer material and locating such vessel so that a first portion thereof is positioned within the oil flow and a second portion thereof is positioned remotely from the oil flow, said portions being arranged so that the heat transfer material in the vessel can flow from the first portion to the second portion thereof; and heating the second portion of the vessel in thermal isolation from said heating oil to wholly vaporize at a temperature less than said predetermined temperature the heat transfer material in said vessel whereby the vapor transfers heat to the first portion of the vessel and thereby to the oil at a temperature at least substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determined within the closed vessel.

2. An oil heating system comprising a closed circuit containing oil to be heated, said oil commencing degradation at a predetermined temperature; an oil circulating pump in said circuit; a chamber 'in said circuit through which oil flows; an array of tubes, each of said tubes having its ends sealed to provide a closed vessel and each said tube extending in a sealed manner through a thermally non conducting wall of said chamber to locate one end portion thereof in the oil in said chamber and the other end portion thereof at a position outside of the chamber and remote from the oil; heater means for heating said other end portions of the tubes; and wherein each said closed vessel formed by the tube contains a heat transfer material which is wholly vaporized in the closed vessel at a temperature less than said predetermined temperature when said other end portion of the tube is heated by the heater means, said vapor serving to transfer heat to said one end portion of the tube by which oil flowing through the chamber is heated to a temperature substantially consistent with the temperature of latent heat of vaporization of the heat transfer material under the pressure conditions determined within said closed vessels.

3. A method of heating a heat degradable heating liquid comprising oil which flows through a chamber, said heating liquid commencing degradation at a predetermined temperature, and which method includes the steps of positioning in the chamber a closed vessel containing a heat transfer material and locating such vessel so that a first portion thereof is positioned within the heating liquid flow and a second portion thereof is positioned remotely from the heating liquid, said'portions being arranged so that the heat transfer material in the vessel can flow from the first portion to the second portion thereof; and heating the second portion of the vessel in thermal isolation from said heating liquid to wholly vaporize at a temperature less than said predetermined temperature the heat transfer material in said vessel whereby the vapor transfers heat to the first portion of the vessel and thereby to the heating liquid at a temperature at least substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determined within the closed vessel.

4. A heating system having a closed circuit containing a heat degradable heating liquid comprising oil to be heated, said liquid commencing degradation at a predetermined temperature; a circulating pump in said circuit; a chamber in said circuit through which said heating liquid flows; an array of tubes, each of said tubes having its ends sealed to provide a closed vessel and each said tube extending in a sealed manner through a thermally non-conducting wall of said chamber to locate one end portion thereof in the liquid in heater means, said vapor serving to transfer heat to said one end portion of the tube by which liquid flowing through the chamber is heated to a temperature substantially consistent with the temperature of latent heat of vaporization of the heat transfer material under the pressure conditions determined within said closed vessels. 

1. A method of heating oil which flows through a chamber, said oil commencing degradation at a predetermined temperature and which method includes the steps of positioning in the chamber a closed vessel containing a heat transfer material and locating such vessel so that a first portion thereof is positioned within the oil flow and a second portion thereof is positioned remotely from the oil flow, said portions being arranged so that the heat transfer material in the vessel can flow from the first portion to the second portion thereof; and heating the second portion of the vessel in thermal isolation from said heating oil to wholly vaporize at a temperature less than said predetermined temperature the heat transfer material in said vessel whereby the vapor transfers heat to the first portion of the vessel and thereby to the oil at a temperature at least substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determined within the closed vessel.
 2. An oil heating system comprising a closed circuit containing oil to be heated, said oil commencing degradation at a predetermined temperature; an oil circulating pump in said circuit; a chamber in said circuit through which oil flows; an array of tubes, each of said tubes having its ends sealed to provide a closed vessel and each said tube extending in a sealed manner through a thermally non conducting wall of said chamber to locate one end portion thereof in the oil in said chamber and the other end portion thereof at a position outside of the chamber and remote from the oil; heater means for heating said other end portions of the tubes; and wherein each said closed vessel formed by the tube contains a heat transfer material which is wholly vaporized in the closed vessel at a temperature less than said predetermined temperature when said other end portion of the tube is heated by the heater means, said vapor serving to transfer heat to said one end portion of the tube by which oil flowing through the chamber is heated to a temperature substantially consistent with the Temperature of latent heat of vaporization of the heat transfer material under the pressure conditions determined within said closed vessels.
 3. A method of heating a heat degradable heating liquid comprising oil which flows through a chamber, said heating liquid commencing degradation at a predetermined temperature, and which method includes the steps of positioning in the chamber a closed vessel containing a heat transfer material and locating such vessel so that a first portion thereof is positioned within the heating liquid flow and a second portion thereof is positioned remotely from the heating liquid, said portions being arranged so that the heat transfer material in the vessel can flow from the first portion to the second portion thereof; and heating the second portion of the vessel in thermal isolation from said heating liquid to wholly vaporize at a temperature less than said predetermined temperature the heat transfer material in said vessel whereby the vapor transfers heat to the first portion of the vessel and thereby to the heating liquid at a temperature at least substantially consistent with the temperature of the latent heat of vaporization of the heat transfer material under the pressure conditions determined within the closed vessel.
 4. A heating system having a closed circuit containing a heat degradable heating liquid comprising oil to be heated, said liquid commencing degradation at a predetermined temperature; a circulating pump in said circuit; a chamber in said circuit through which said heating liquid flows; an array of tubes, each of said tubes having its ends sealed to provide a closed vessel and each said tube extending in a sealed manner through a thermally non-conducting wall of said chamber to locate one end portion thereof in the liquid in said chamber and the other end portion thereof at a position outside of the chamber and remote from the said liquid; heater means for heating said other end portions of the tubes; and wherein each said closed vessel formed by the tube contains a heat transfer material which is wholly vaporized in the closed vessel at a temperature less than said predetermined temperature when said other end portion of the tube is heated by the heater means, said vapor serving to transfer heat to said one end portion of the tube by which liquid flowing through the chamber is heated to a temperature substantially consistent with the temperature of latent heat of vaporization of the heat transfer material under the pressure conditions determined within said closed vessels. 